Data from Antarctic ice cores show that temperatures have changed before CO2 concentrations over a series of recent ice ages. That trend has been upset during the past 100 years, as a rapid increase in CO2 preceded the current warming.

Credit: Michael Ernst/Woods Hole Research Center

Lead Versus Lag

Data from Antarctic ice cores show that temperatures have changed before CO2 concentrations over a series of recent ice ages. That trend has been upset during the past 100 years, as a rapid increase in CO2 preceded the current warming.

Credit: Michael Ernst/Woods Hole Research Center

Global-warming skeptics have developed a set of talking points to use in their arguments as they lobby against anthropogenic global warming. Many of these points fall into the category of "climate canards." These urban legends and sometimes outright misinformation add confusion to public understanding of climate science and have been refuted by the mainstream climate research community over the years. Yet they keep popping up.

One canard is that the temperature increase signaling the end of an ice age is observed to come before an increase in carbon dioxide concentration, rather than after the increase. This observation seems to be a contradiction in global-warming theory: Does an increase in CO2 concentration drive temperature rise, or is it the other way around?

The answer is both, although intuitively many people assume that it can only be one way or the other, notes geophysicist Michael E. Mann, director of Pennsylvania State University's Earth System Science Center. Mann is part of a group of climate scientists who run the website "RealClimate," which provides news and commentary on global warming and climate change to counter the blogs operated by skeptics.

Ice-core data on CO2 concentrations stretching back over the past 800,000 years and confirmed by geological evidence show that Earth's climate has been subject to long ice ages broken by short, warm interglacial periods. As currently understood, this cycle takes place about every 120,000 years and is caused by regular, predictable changes in Earth's orbit around the sun—Earth moves closer and farther away from the sun—and shifts on its axis, Mann explains.

As these changes occur, Earth begins warming, which melts ice and snow so that the planet absorbs more heat, which serves as a feedback to increase the warming. Eventually, the oceans warm enough to start giving off CO2. The CO2 subsequently spreads throughout the atmosphere, absorbing more sunlight and trapping heat to amplify the warming effect.

A related canard is that CO2's warming effect is limited because at higher concentrations it saturates the atmosphere and can absorb only so much solar radiation or heat from Earth's surface. Global warming skeptics have used this argument to erroneously explain why Earth stops warming up after an ice age. But the reality is that CO2 disperses to other layers of the troposphere—the greenhouse blanket becomes thicker—so that the absorption capacity is not diminished, Mann says.

So it's a fact that increases in CO2 follow increases in temperature by up to about 1,000 years during interglacial periods, Mann says. This natural increase in CO2 helps bring the planet out of ice ages and moderates temperatures during interglacial periods, such as today, he points out. This cycle also gives climate scientists confidence in predicting that anthropogenic CO2 added to the natural CO2 levels will lead to more warming, Mann adds.

But atmospheric physicist and prominent global-warming skeptic S. Fred Singer says there is still a cause-and-effect problem with that CO2 analysis, and not just for ice ages, but throughout the Intergovernmental Panel on Climate Change's (IPCC) assessments on the causes and effects of human-caused global warming. If the theory says CO2 drives temperature change, then a rise in CO2 should always come before a temperature increase, not after it, Singer says.

Singer's complete counterargument is laid out in "Climate Change Reconsidered," a report issued by the Nongovernmental International Panel on Climate Change, which was established by global-warming skeptics to oppose IPCC. For Singer, the lack of consistent CO2-temperature correlation invalidates global-warming theory, and he says it's one example of "insufficient evidence" by IPCC in proving its case for anthropogenic global warming.